The drone ship uses precision GNSS positioning and four large thrusters to maintain its position and attitude. The returning rocket aims for the geographic location of the drone ship landing zone. It really is quite amazing that it works. In an interview, Elon Musk said they had tried it three times. The first two attempt failed, but from each of those failures they learned how to compensate for the problem. They got it to work on the third try. Looks like it worked again on the fourth try.
Here is a much better link to the story:
http://www.space.com/32811-spacex-rocket-landing-jcsat-14-launch.htmlThis landing looks like it was only a few feet off dead-center of target. These FALCON rocket and drone ship landings have been very widely covered, and many good videos of the daylight landing (attempt three) are available.
SpaceX was the first private rocket company to lift a payload to the International Space Station. By recovering the booster rocket, SpaceX hopes to be able to re-use them, which could cut the cost of launching payloads into orbit.